1. Field of the Invention
This invention relates to novel negative photoresist compositions and to the production of negative photoresist layers. In particular, the invention deals with a novel class of fluorene polymers that when cast in thin films, form crosslinked networks after exposure to radiation such as ultraviolet light, making the materials useful as negative photoresists.
2. DESCRIPTION OF THE PRIOR ART
Photoresists are materials whose solubility in a developer solution is altered after the photoresist has been exposed, to a source of radiation. Photoresist compositions may comprise a photosensitive compound (hereafter sometimes called a sensitizer or photosensitizer) which is blended with a film forming polymeric resin and a solvent. Photoresist compositions may also comprise polymers which, of themselves, are inherently light sensitive. It is with the latter that the present invention is concerned. As a consequence of the exposure to radiation of the photoresist (usually in the form of a thin film), a different solubility rate results between the exposed and unexposed (masked over) portions of the resist film which yields a surface relief pattern after the development. Those photoresists whose solubility is diminished in the exposed regions are referred to as negative photoresists.
The photoresists are applied in any suitable manner, such as by spin coating from an organic solvent or solvent mixture, onto a substrate, such as silicon wafers and chrome plates glass plates. A developer removes the areas of the coated photoresist film that has been exposed to light or other form of irradiation so as to produce a pattern in the photoresist film.
The application of the photosensitive film to various substrates is an essential step in the fabrication of integrated circuits. The substrates are generally silicon wafers which may have a thin oxide coating or other coating such as a silicon nitride or aluminum. The photosensitive film is used to pattern the substrate in a series of steps including exposure, development and substrate etch. It is essential that the mask pattern be accurately reproduced in substrate etch pattern. To achieve this high degree of accuracy, it is essential that the photoresist film be of uniform thickness, have good adhesion to substrates, good contrast in images formed, and good etch resistance properties.
One of the limitations of materials currently used as commercial photoresists is their lack of sensitivity to light at wavelengths much below 300 nm. The resolution attainable with these resists is typically in the 2-4.mu. range and would be improved were they sensitive to shorter wavelengths of light. There is, therefore, a need for photoresist materials that are sensitive in the deep ultraviolet (UV) light range (200-300 nm), in that such materials permit a higher resolution and thus a higher information density in the microcircuits manufactured using them. This sensitivity in the deep UV light range should of course be accompanied by the other normal attributes, including good contrast in images formed, and good etch resistance properties.